During the operation of lead acid storage batteries, especially during the final stages of the charging operation, hydrogen and oxygen are freed which are usually vented to the atmosphere by means of a venting device provided on top of the battery casing. Unless such device is equipped with a reliable flame barrier the escaping gases constitute an imminent explosion hazard whenever an open flame, or a spark comes into contact with these gases. Furthermore, the said gases usually carry with them a certain quantity of acid thus making them corrosive.
Hitherto protection against explosion was mainly provided by one of the following means:
A. a bell shaped body of porous material covering the vent hole completely, and which therefore had to be removed for maintenance operations, leaving the hole unprotected during such operations PA1 B. an open filling funnel whose bottom aperture is always below the electrolyte level. This funnel is surrounded by a large bushing made of porous material. The gases enter the space between the funnel and the inside of the bushing and are free to diffuse through its body to the atmosphere.
In both cases the porous material acts as a flame barrier. The drawbacks of these arrangements are that the porous material, being usually slightly moist, is likely to become covered with a layer of dust thus clogging the pores. Dust and foreign matter might also enter the open funnel.